JP2000261098A - Self-oscillating type semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a self-oscillating type semiconductor laser, the saturable absorption layer of which is not affected by carrier overflows and which makes stable self-pulsating operations up to a high temperature. SOLUTION: In a semiconductor laser, having a double heterostructure composed of an n-type clad layer 103, an MQW active layer 105, and a p-type clad layer 106, an n-type saturable absorption layer 104 having a compressive strain, is provided in the n-type clad layer 103. Consequently, such a self- pulsation type semiconductor layer can be realized that can make stable self- oscillating operations up to a high temperature, in a state where the saturable absorption layer 104 is not affected by carrier overflows.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser used as a light source for an optical disk system or the like, and more particularly to a self-pulsation type semiconductor laser which is strong against return light from an optical disk or the like.

[0002]

2. Description of the Related Art Digital versatile discs (DVs)
D) An AlGaInP-based visible light semiconductor laser is used as a light source for an optical disk system such as a magneto-optical (MO) disk. What is important in such semiconductor lasers for optical disks is noise characteristics. In particular, return noise generated when the output laser light is reflected by an optical disk or the like and returned to the semiconductor laser itself is important, and since return light noise causes a signal reading error, its reduction is indispensable. Has become.

Conventionally, a self-pulsation type semiconductor laser has been known as a semiconductor laser resistant to return light noise. In particular, a structure in which the active layers on both sides of the ridge waveguide have a saturable absorption region and a structure in which a saturable absorption region is provided in the resonator direction have been proposed as measures to reduce noise.

Further, a semiconductor laser aimed at obtaining a stable self-sustained pulsation operation at a high temperature is disclosed in Japanese Patent Application Laid-Open No. 10-2566.
No. 42 discloses this. In this semiconductor laser,
The material of the active layer and the material of the saturable absorption layer are selected such that the rate of change of the light absorption coefficient with respect to the absorption wavelength of the saturable absorption layer is substantially saturated near the oscillation wavelength from the active layer. It is intended to achieve the excitation oscillation.

[0005] As another conventional example, the p in the vicinity of the active layer is reduced.
A self-pulsation type semiconductor laser having high controllability in manufacturing and small astigmatism, having a saturable absorption layer having a band gap close to the energy of oscillation light in a cladding layer, is described in Japanese Patent Application Publication No.
Proceedings of the Japan Society of Applied Physics (October 1997
Mon., 3rd volume, 1126), I. Kidoguchi, H. Adachi,
S.Kamiyama, T.Fukuhisa, M.Mannoh, and A.Takamor
i: “Low-Noise 650-nm-Band AlGaInP Visible Laser D
iodes with a Highly Doped Saturable Absorbing Laye
r ”(IEEE Journal of Quantum Electronics, Vol. 33,
No. 5, pp. 831-837, MAY 1997).

[0006]

However, the semiconductor laser disclosed in Japanese Unexamined Patent Application Publication No. 10-256,642 has a problem in that the material of the active layer and the material of the saturable absorption layer are changed each time according to the required specifications. You must select a combination. Therefore, there is a case where there is no material combination that satisfies the required specifications, which is a great limitation in designing and manufacturing.

[0007] In general, an optical disk system generally has 6
Operation guarantee is required up to a temperature range of 0 ° C. or more (about 70 to 80 ° C. for a vehicle). It has been reported that a self-pulsation type semiconductor laser provided with a saturable absorption layer in a p-cladding layer near an active layer maintains self-pulsation up to a temperature of about 60 ° C. About 80 ℃)
It is difficult to maintain self-sustained pulsation until. That is, AlGaInP
In a system-based semiconductor laser, the hetero barrier between the active layer and the p-cladding layer is small, and carrier overflow from the active layer becomes significant at high temperatures. Therefore, a large amount of electrons overflowing from the active layer at a high temperature flow into the saturable absorption layer in the p-cladding layer and accumulate. As a result, the saturable absorbing layer becomes transparent and always saturated with respect to the oscillating light, and it becomes difficult to maintain the self-excited oscillating operation at a high temperature.

An object of the present invention is to provide a self-pulsation type semiconductor laser in which a saturable absorption layer is not affected by carrier overflow and performs stable self-pulsation operation up to a high temperature.

[0009]

According to the present invention, there is provided a self-pulsation type semiconductor laser in which a double hetero structure comprising at least an n-type cladding layer, an active layer and a p-type cladding layer is formed on a substrate. An excited oscillation type semiconductor laser, characterized in that a saturable absorbing layer made of an n-type semiconductor and having a compressive strain is provided in the n-type cladding layer. n
By providing the saturable absorption layer made of the n-type semiconductor in the type clad layer, the hetero barrier on the side of the n-type clad layer becomes larger than that on the side of the p-type clad layer, and is less affected by carrier overflow. This makes it possible to obtain a stable self-excited oscillation operation up to a high temperature. In addition, since the saturable absorbing layer has a compressive strain, the state density of minority carriers can be reduced, saturability can be further easily caused, and a stable self-excited oscillation operation up to a high temperature can be obtained.

[0010]

FIG. 1 is a schematic perspective view showing a first embodiment of a self-pulsation type semiconductor laser according to the present invention. This embodiment uses the MOVPE method (Met
al Organic Vaper Phase Epitaxial growth system (organic metal vapor phase epitaxy).

First, in the first crystal growth step by the MOVPE method, the following layers are formed on an n-GaAs substrate 101 having a (115) A orientation (15.8 degrees off from (001) to [110] direction). The layers are sequentially formed. That is, as shown in FIG.
GaAs buffer layer 102, 1.0 μm thick n-AlGaInP cladding layer 103a, n-AlGaInP saturable absorption layer 104,.
A 15 μm thick n-AlGaInP cladding layer 103b, MQW (Multi
ple Quantum Well) The active layers 105, 0.
25 μm thick p-AlGaInP cladding layer 106, 5 nm thick p
-GaInP etching stopper layer 107, 0.95 μm thick
A p-AlGaInP clad layer 109, a 10 nm thick p-GaInP heterobuffer layer 110, and a 0.3 μm thick p-GaAs cap layer 111 are stacked in this order.

Here, the MQW active layer 105 is formed of, for example, 5
GaInP well layer with nm thickness, +0.5 compressive strain and 4n
m of AlGaInP barrier layer.

The band gap of the n-AlGaInP saturable absorption layer 104 is set to be substantially equal to the energy of the laser oscillation light or smaller than the energy of the laser oscillation light.
The n-AlGaInP saturable absorption layer 104 may be composed of a well layer that becomes saturable absorption and a guide layer for enhancing light confinement in the well layer. Compressive strain is applied to the well layer, and the amount of compressive strain is assumed to be within the critical film thickness. As an example, the well layer is made of n-Ga having a thickness of 5 nm and a compressive strain of +0.9.
Consists of InP. The guide layer is, for example, a 50 nm thick n.
-It is composed of AlGaInP.

Subsequently, a mesa stripe-shaped ridge structure is formed by etching using a dielectric mask, and thereafter, a selective growth method (second crystal growth step) using the dielectric mask is used to form a ridge structure outside the mesa stripe. An n-GaAs current blocking layer 108 having a thickness of 1.0 μm is formed on the cladding layer.

Finally, the dielectric is removed, and in the third crystal growth step, a p-GaAs contact layer 112 having a thickness of 3.0 μm is formed on the entire surface to complete the growth. Thereafter, a laser is manufactured through backside polishing and an electrode process for forming the n-electrode 114 and the p-electrode 113. The laser structure may be a double growth type in which the third crystal growth step is omitted.

The self-sustained pulsation type semiconductor laser produced by the above-described process has an n-AlGaInP saturable absorption layer 104 sandwiched between n-AlGaInP cladding layers 103a and 103b. Therefore, the MQW active layer 105 and the n-AlGaInP cladding layer 103b
Between the MQW active layer 105 and p-AlGaInP
It is larger than the hetero barrier between the cladding layer 106. Specifically, the hetero barrier on the n-AlGaInP cladding layer 103b side: the hetero barrier on the p-AlGaInP cladding layer 106 side = 6: 4. Further, since the effective mass of holes, which are minority carriers, in the MQW active layer 105 is several times larger than the electrons and the state density is large, the holes in the MQW active layer 105 do not distribute to a high energy region. For these reasons, even at a high temperature, the flow of carriers from the MQW active layer 105 to the n-AlGaInP saturable absorption layer 104 is suppressed, and overflow does not easily occur.

In order to facilitate self-sustained pulsation, it is necessary to reduce the state density of minority carriers in the n-AlGaInP saturable absorber layer 104 to facilitate saturable absorption of oscillation light. However, the effective mass of holes, which are minority carriers, in the saturable absorption layer is several times larger than that of electrons, and the holes are difficult to be distributed to high energies, so that saturability is difficult. To avoid this problem, n-
A compressive strain is applied to the AlGaInP saturable absorption layer 104. As a result, the energy band structure is changed, the density of states of holes as minority carriers is reduced, and holes are distributed to a high energy region. In addition, the band gap of the n-AlGaInP saturable absorption layer 104 is substantially equal to or smaller than the energy of the laser oscillation light, so that saturability due to the absorption of the oscillation light easily occurs.

As described above, the n-AlGaInP cladding layer 103
Even in the configuration in which the n-AlGaInP saturable absorption layer 104 is provided, self-sustained pulsation easily occurs easily, is not affected by carrier overflow, has high controllability in manufacturing, and is stable up to a high temperature region. A self-pulsation type semiconductor laser capable of self-pulsation can be obtained. Specifically, the oscillation wavelength 6
In the 50 nm band, self-sustained pulsation can be maintained up to 90 ° C., and a practically available AlGaInP-based self-sustained pulsation semiconductor laser was obtained.

FIG. 2 is a schematic perspective view showing a second embodiment of the self-pulsation type semiconductor laser according to the present invention. The self-pulsation type semiconductor laser according to the second embodiment has the same double heterostructure as the first embodiment, and the same reference numerals are given to the same materials and layers having the same functions, and description thereof is omitted. .

The second embodiment differs from the first embodiment in the structure of the current blocking layer. In the second embodiment,
The n-GaAs current block layer 108 of FIG.
The configuration is changed to a combination of an InP current block layer 201 and an n-GaAs current block layer 202 having a thickness of 0.7 μm. Specifically, as shown in FIG. 2, the AlInP current blocking layer 2
Numeral 01 covers a part of the upper surface of the p-GaInP etching stopper layer 107 and side surfaces of the p-AlGaInP cladding layer 109, the p-GaInP heterobuffer layer 110, and the p-GaAs cap layer 111.

The n-GaAs current blocking layer 108 used in the first embodiment shown in FIG. 1 absorbs oscillation light and has a large loss. On the other hand, since the AlInP current blocking layer 201 used in the second embodiment is transparent to the oscillating light, the mode loss for laser oscillation can be reduced.

To obtain self-sustained pulsation, it is effective to reduce the ratio of the mode loss to the loss of the saturable absorber layer. Use of an AlInP transparent waveguide facilitates self-sustained pulsation. As a result, the self-sustained pulsation can be maintained up to a high temperature region as in the first embodiment, and the self-sustained pulsation can be realized in a wider operation range than in the first embodiment.

In the above, the AlInP current blocking layer 20
In order to avoid surface oxidation or the like after the growth of 1, the thickness of the AlInP current block layer 201 is suppressed to a thickness that allows the laser oscillation light to exude, and an n-GaAs current block layer 202 is formed thereon as a protective layer. did.

FIG. 3 is a band structure diagram for explaining a difference in hetero barrier when the saturable absorption layer 104 is composed of a well layer and a guide layer. However, for the sake of simplicity, a band structure in an undoped and unbiased state is shown.

As shown in FIG. 1, an n-AlGaInP saturable absorbing layer 104 is formed on an n-AlGaInP cladding layer 103a. The saturable absorbing layer 104 is formed by a well layer 104a serving as a saturable absorbing layer. And a guide layer 104b for enhancing light confinement in the well layer 104a. Specifically, the well layer 104a has a thickness of 5 nm and a compressive strain of +0.9.
The guide layer 104b is made of n-GaInP and has a thickness of 50 nm.
Of n-AlGaInP. The band gap of the well layer 104a is set substantially equal to the energy of the laser oscillation light or smaller than the energy of the laser oscillation light.

As described above, the well layer 104a and the guide layer 104 are provided on the n-clad layer 103a side of the MQW active layer 105.
By forming the saturable absorption layer 104 made of b, as described above, the light confinement effect is enhanced, and the hetero barrier on the n-cladding layer 103b side of the MQW active layer 105 becomes p-type.
-Larger than the cladding layer 106 side,
The effective mass of holes, which are minority carriers, in the active layer 105 is several times larger than that of electrons, and the state density is large.
Holes in the W active layer 105 do not distribute to a high energy region. Therefore, even at a high temperature, the flow of carriers from the MQW active layer 105 to the saturable absorption layer 104 is suppressed, and overflow does not easily occur.

The first embodiment and the second embodiment use Al
The GaInP system was explained, but other compound semiconductor materials,
For example, a similar effect can be expected in an AlGaAs or AlGaInN system.

[0028]

As described above, according to the present invention,
By providing a saturable absorbing layer having a compressive strain in the n-type cladding layer, the hetero barrier between the active layer and the n-type cladding layer is made larger than the hetero barrier between the active layer and the p-type cladding layer. Even at a high temperature, the flow of carriers from the active layer to the saturable absorbing layer is suppressed, and self-sustained pulsation easily occurs. Further, when compressive strain is applied to the saturable absorption layer, the energy band structure changes, and the density of states of holes, which are minority carriers, decreases. As a result, saturability is more likely to occur, and self-excited oscillation is more likely to occur.

Further, by using a semiconductor material (for example, AlInP) transparent to oscillation light for the current blocking layer, the ratio of the mode loss to the loss of the saturable absorbing layer can be reduced. As a result, self-sustained pulsation can be easily obtained, and self-sustained pulsation can be realized in a wider operation range.

[Brief description of the drawings]

FIG. 1 is a schematic perspective view showing a first embodiment of a self-pulsation type semiconductor laser according to the present invention.

FIG. 2 is a schematic perspective view showing a second embodiment of the self-pulsation type semiconductor laser according to the present invention.

FIG. 3 is a band structure diagram for explaining a difference in hetero-barrier when the saturable absorption layer 104 is composed of a well layer and a guide layer.

[Explanation of symbols]

 101 n-GaAs substrate 102 n-GaAs buffer layer 103a, 103b n-AlGaInP cladding layer 104 n-AlGaInP saturable absorption layer 105 MQW active layer 106 p-AlGaInP cladding layer 107 p-GaInP etching stopper layer 108 n-GaAs current block Layer 109 p-AlGaInP cladding layer 110 p-GaInP hetero buffer layer 111 p-GaAs cap layer 112 p-GaAs contact layer 113 p electrode 114 n electrode 201 AlInP current blocking layer 202 n-GaAs current blocking layer

Claims (9)

[Claims] 1. At least an n-type cladding layer on a substrate,
In a self-pulsation type semiconductor laser in which a double hetero structure composed of an active layer and a p-type cladding layer is laminated, a saturable absorption layer composed of an n-type semiconductor and having compressive strain is provided in the n-type cladding layer. A self-pulsation type semiconductor laser characterized by the above-mentioned. 2. A band gap of the saturable absorbing layer,
2. The self-excited oscillation type semiconductor laser according to claim 1, wherein the oscillation wavelength energy of the laser oscillation light is substantially equal to or smaller than the oscillation wavelength energy of the laser oscillation light. 3. The self-pulsation type semiconductor laser according to claim 1, further comprising a guide layer for enhancing light confinement in the saturable absorption layer. 4. The self-pulsation type semiconductor laser according to claim 1, wherein an amount of the compressive strain of the saturable absorption layer is within a critical film thickness. 5. The substrate is made of n-type GaAs, and the n-type clad layer made of n-type AlGaInP, the active layer made of multiple quantum wells, and the p-type clad layer made of p-type AlGaInP are formed on the substrate. 5. The self-pulsation type semiconductor laser according to claim 1, which has a double hetero structure formed in this order. 6. At least an n-type cladding layer on a substrate,
In a self-pulsation type semiconductor laser having a double hetero structure including an active layer and a p-type cladding layer, wherein the p-type cladding layer has a mesa stripe shape, the n-type cladding layer may include an n-type semiconductor. A self-sustained pulsation type semiconductor laser comprising a saturation absorption layer, and both sides of the mesa stripe-shaped p-type cladding layer are sandwiched between current blocking layers. 7. The self-pulsation type semiconductor laser according to claim 6, wherein said current blocking layer is made of a semiconductor material transparent to oscillation light. 8. The transparent semiconductor material comprises AlGaInP and A
The self-pulsation type semiconductor laser according to claim 7, wherein the self-pulsation type semiconductor laser is a material selected from lInP. 9. The self-pulsation type semiconductor laser according to claim 7, wherein the thickness of the current blocking layer is about the amount of exudation of laser oscillation light.